This invention relates to furnaces for melting thermoplastic materials such as glass, and more particularly to those furnaces having a protective liner within the melting chamber of the furnace for protecting the sidewalls and/or the bottom walls of the melting chamber from the corrosive action of the melt. As pointed out in U.S. Pat. No. 4,366,571, which is a basic patent relating to the use of a protective liner in a melting furnace, the use of a refractory metal liner protects the refractory sidewalls and bottom of the melting chamber from the corrosive action of the molten bath contained therein. However, since the liner was preferrably formed from an oxidizable refractory metal, in order to protect the liner from oxidation, it was immersed in the molten bath during normal operations, with the upper end terminating below the fusion line.
FIG. 1 illustrates the prior art device such as shown in U.S. Pat. No. 4,366,571. As illustrated, the prior art furnace 10 included an outer shell 12 having sidewalls 14 and a bottom wall 16, which may be electrically insulated by insulating shims 18. A refractory vessel 20, having upstanding sidewalls 22 and a bottom wall 24, and which may be formed from a plurality of refractory blocks, is retained within the outer shell 12 and is preferrably provided with a plurality of layers of insulation 26, 28 between the sidewalls 22 of the vessel 20 and the sidewalls 14 of the outer shell 12. A liner 30 is positioned within the refractory vessel 20 in closely spaced-apart relationship with respect to the sidewalls 22 of the vessel so as to form a limited space 32 between the liner and such sidewalls. In addition, the liner 30 has an outwardly directed flange 34 formed below its upper margin 36, which flange associates with an outwardly stepped ledge 38 of refractory sidwall 22, forming an offset trap 40 behind the upper portion of the liner 30.
In view of the fact that the liner 30 is preferrably made of a refractory metal material which is oxidizable at elevated temperatures, such as molybdenum, it has been considered imperative to maintain the upper margin 36 of the liner below the glass line or fusion line 42, extending between the molten bath 44 within the vessel 40 and the batch material 46 supplied to the vessel. Further, the space 32 and the trap 40 are supplied with cullet or other suitable materials which become thermoplastic or semi-molten during the furnace operation, to not only encompass the outer portions of the liner to protect it from oxidation, but also provide substantially quiescent zones for protecting the refractory walls of the vessel 20 from corrosion.
In a like manner, U.S. Pat. No. 3,109,045 discloses the use of a refractory metal melting pot which is immersed in a bath of molten glass within a tank, such that the upper edge of the melting pot is below the level of the glass in order to protect it from oxidation. A refractory member seats upon the upper rim or top surface of the melting pot and provides a seal which prevents the entrance of the surrounding molten glass into the melting chamber.
U.S. Pat. No. 2,777,254 relates to a coated refractory for contacting molten glass, wherein each block of refractory is provided with a protective coating. That is, platinum foil is bonded by heat and pressure and alumina to refractory blocks, such as zirconium or the like, and such platinum-bonded refractory is utilized in glass contact portions of a molten glass tank. However, in view of the fact that the platinum is a precious metal and is not readily oxidizable, it may be positioned both within and without the molten bath.
United Kingdom Patent Specification No. 601,851 relates to electrically heated glass melting furnaces wherein the refractory walls of the furnace are lined with iron plates which extend through the glass line to the top of the furnace, where the batch charge protects the iron from oxidation. However, the iron plates extend only to the glass line in the working chamber of the furnace, where no protective batch blanket exists.
German Pat. No. 1,230,177 discloses the use of molybdenum baffles about the sidewalls of the furnace relatively close to, but below the melt line, so as to protect the molybdenum baffles from oxidation and the refractory sidewalls from the corrosive action of the glass adjacent the glass line.
As previously mentioned with respect to FIG. 1, the liner 30 was protected from corrosion by not only maintaining its upper extent 36 below the fusion line 42, but also by providing a radial flange 34 and an offset 38 so as to allow the glass within the trap 40 above the flange to cool enough so that it may contact the vertical refractory wall 22 without excessive corrosion of the wall. Further, it was deemed necessary to terminate the upper margin 36 of the liner 30 below the fusion line 42 so that the liner would not extend into and be corroded by the corrosive batch materials 46.
However, it has been discovered that many batch materials are not corrosive with respect to molybdenum and other refractory metals of which the liner may be formed. That is, it appears that the corrosion of molybdenum in the batch blanket is proportional to the amount of oxidizing gases produced by the melting batch materials. Hence, the proposed embodiments may be utilized with batch materials which produce small amounts of oxidizing gases. Such batch materials, which are not corrosive to molybdenum and the like, do not contain large amounts of carbon dioxide, or other materials which produce sufficient oxidizing gases to be detrimental to the molybdenum. Accordingly, when melting glasses such as borosilicates, the batch of which contains very few carbonates, the liner may be extended upwardly so as to be immersed in the batch blanket without suffering any deleterious corrosion effects. It is preferrable to provide the upper portion of the liner with an effective seal with the refractory wall so as to minimize the introduction of batch materials behind the liner. Such batch materials could create fresh glass between the liner and the refractory, especially during start-up operations of the furnace, which glass could contribute to the corrosion of the refractory behind the liner.
The present invention overcomes the problems encountered with known furnace liners by providing a liner which not only protects the refractory wall behind the liner in the molten bath from corrosion, but also functions to protect the refractory wall above the liner from corrosion. In addition, the upper portion of the liner is terminated above the fusion line without deleterious thermal or corrosive effects to the liner per se, and the upper portion of the liner is effectively sealed to the refractory vessel so as to minimize the formation of fresh glass behind the liner which would promote corrosion of the refractory.